Oil country tubular goods (abbreviated as OCTG) are tubing and casing which are used to excavate oil wells for recovering crude oil or gas oil. They are normally connected to each other using threaded joints. In the past, the depth of oil wells was 2,000-3,000 meters, but in recent deep sea oil fields, it may reach 8,000-10,000 meters.
In its environment of use, a threaded joint for OCTG is subjected to a load in the form of an axial tensile force caused by the weight of OCTG and joints connected thereto, a combined internal and external pressure, and subterranean heat. Therefore, the threaded joint must maintain an airtight connection without breakage even in such a severe environment.
A typical threaded joint used for connecting OCTG has a pin-box structure comprising a male thread formed on the outer surface of each end portion (pin) of an oil country tubular good and a female thread formed on the inner surface (box) of a coupling (a threaded connecting member). An unthreaded metal-to-metal contact portion is formed on the front-end side of the male thread of the pin and on the inner side of the female thread of the box. By inserting one end of the oil country tubular good into the coupling and tightening the male thread and the female thread, the unthreaded metal-to-metal contact portions of the pin and the box are made to tightly contact each other to form a metal-to-metal seal, thereby achieving airtightness of the joint. Thus, each of the pin and box has a contact surface in its threaded portion and its unthreaded metal-to-metal contact portion.
During the operation of lowering tubing or casing into an oil well, due to various problems, there are cases in which a threaded joint which has once been tightened is loosened after tubing or casing being temporarily lifted out of the oil well, and then the joint is retightened before lowering the tubing or casing. API (the American Petroleum Institute) requires a threaded joint for OCTG to have sufficient galling resistance to make it possible to carry out tightening (makeup) and loosening (breakout) ten times for a joint for tubing and three times for a joint for casing without the occurrence of galling (unrepairable severe seizure) while maintaining airtightness.
When tightening a threaded joint, in order to increase its galling resistance and airtightness, a viscous liquid lubricant referred to as “compound grease” which contains a large amount of heavy metal powder is applied to the contact surfaces (namely, the threaded portions and the unthreaded metal-to-metal contact portions) of the joint. Such a compound grease is specified by API BUL 5A2. Compound grease also has the effect of preventing the surface from rusting.
For the purpose of increasing the retention of compound grease by a threaded joint and improving the sliding properties of the joint, it has been proposed that the contact surfaces of a threaded joint be subjected to surface treatment selected from nitride treatment, various types of plating such as zinc-based plating and dispersed plating, phosphate chemical conversion treatment, and the like to form one or more layers having increased surface roughness. However, the use of a compound grease has a concern of adverse effects on the environment and the human body, as described below.
Compound grease contains a large amount of powder of heavy metals such as lead, zinc, and copper in order to provide the grease with sufficient lubricity and corrosion resistance. When tightening a threaded joint having compound grease applied thereto, the applied grease is washed off or squeezed out to the exterior of the joint, and it may have an adverse effect on the environment and particularly on marine life due to its harmful heavy metal constituents such as lead. In addition, the process of applying compound grease worsens the working environment, and there is also a concern of harmful effects on the human body due to the harmful constituents.
Since the OSPAR Convention (Oslo-Paris Convention) pertaining to preventing maritime pollution in the Northeast Atlantic came into effect in 1998, strict environmental regulations have been increasing on a global scale, and in some regions, the use of compound grease is already being regulated. Accordingly, in order to avoid adverse effects on the environment and the human body in the excavation of gas wells and oil wells, a demand has developed for a threaded joint which can exhibit excellent galling resistance without application of a compound grease.
As a threaded joint which can be used for connecting OCTG without application of a compound grease, for example, Patent Document 1 cited below discloses a joint for steel pipes having at least three layers formed by surface treatment comprising a lowermost nitrided layer, an intermediate chemical conversion layer formed by manganese phosphating treatment, and an uppermost solid lubricating coating. Although the object is totally different from the present invention, Patent Document 2 cited below discloses a threaded joint for OCTG having a lower plated layer with a soft metal such as Cu, Zn, Sn, or Pb and an upper plated layer with a hard metal such as Cr, Mo, or W, while Patent Document 3 cited below discloses a threaded joint for OCTG having a lower coating layer of a material such as Ni having a melting point higher than the temperature of use of the joint and an upper coating layer of a material such as Sn having a melting point lower than the temperature of use of the joint.